Detection of aromatic hydrocarbons and incipient particles in an opposed-flow flame of ethylene by spectral and time-resolved laser induced emission

Spectral and time-resolved fluorescence and incandescence measurements are used to identify aromatic compounds and particles in an atmospheric-pressure opposed-flow flame of ethylene. The fourth harmonic radiation, at 266 nm, of a Nd-YAG laser is used to excite electronic transitions in aromatic intermediates resulting in different laser induced emission signals. Time-resolved analysis of the emission spectra is performed to obtain information about the structure of the compounds responsible for emission signals. Four fluorescence bands in the UV and visible range and a continuum with a maximum at larger wavelengths, attributed to incandescence radiation by solid particle, have been identified in the collected spectra. On the basis of fluorescence emission spectra and fluorescence lifetimes, two different types of high-molecular mass aromatic compounds are individuated in the flame: highly-packed, sandwich-like structures (cluster of PAHs held together by van der Waals forces) and more loose aromatic moieties linked together by a-bonds, such as the aromatic-aliphatic linked hydrocarbons. The first class of particles has been found in the fuel zone and across the stagnation plane of the opposed-flow flame. In this region (<1500 K) coagulation of aromatics is the leading mechanism forming high-molecular mass structures. The second class of particles is preferentially formed in the region close to the oxidizer side characterized by high temperatures (>1500 K) and growth of aromatics enhanced by the high concentration of small radicals and gas-phase polycyclic aromatic hydrocarbons.